1. Field of the Invention
This invention relates to cryostats utilized to produce and maintain an inventory of a liquified cryogen such as helium. The inventory of liquified cryogen can be used as a low temperature environment for such things as infra-red detectors and super-conducting devices. Such devices require extremely low temperature environments for efficient operation. Temperatures on the order of 4.2 degrees Kelvin (K.) (-268.9 degrees C.) are necessary to maintain helium in the liquified state. Without supplementary refrigeration a bath of liquid helium would, due to heat infiltration even in the best insulated Dewar, boil off and be lost. Attempts have been made to combine a refrigerator with a reservoir or Dewar to prevent excessive loss of liquid helium and thus maintain the overall level of inventoried helium.
2. The Prior Art
Cryostats for producing and maintaining an inventory of a liquified cryogen such as helium range from miniature systems such as disclosed in U.S. Pat. No. 3,728,868 to large systems such as shown in U.S. Pat. Nos. 2,458,894 and 3,360,955.
The miniature system of the '868 patent employs a source of high-pressure gas cooled through heat exchange and expanded through a Joule-Thompson valve to provide a small inventory of liquid helium at the bottom of a glass Dewar.
The '894 patent discloses a system employing one or more expansion engines and multi-stage heat exchangers combined with Joule-Thompson expansion valves to produce liquid helium for use in heat exchange with air to liquify the air for subsequent fractional distillation to produce, inter alia, oxygen.
The '955 patent also employs one or more expansion engines, multi-stage heat exchangers, and Joule-Thompson expansion valves to produce liquid helium to cool various types of electrical apparatus including super-conducting devices.
In relation to super-conducting electronic devices such as super-conducting magnets, super-conducting quantum interference devices and Josephson Junction Devices, applicant in copending application Ser. No. 17,009 filed Mar. 2, 1979 discloses a cryostat wherein the refrigerator can be removed from the vacuum housing, for routine maintenance or in the event the refrigerator malfunctions, without disturbing the vacuum. As pointed out in the copending application, it is desirable to have a cryogenic refrigerator as part of the cryostat so that possible points of heat infiltration (e.g. neck tube accessing the liquid cryogen inventory) can be cooled to prevent heat infiltration and to recondense normal cryogen boil-off from the liquid reservoir.
One cryogenic refrigerator ideally suited for this application is sold as a DISPLEX Model CS-308 Closed Cycle Helium Refrigeration System by Air Products and Chemicals, Inc., Allentown, Pennsylvania. U.S. Pat. No. 3,620,029 discloses in its specification the displacer-expander refrigerator portion of the model CS-308 refrigeration system. This type of refrigerator has been used in many applications. In Mossbauer Spectroscopy it is essential to isolate the sample holder from vibration. Therefore, non-contact transfer of refrigeration by circulating gas as disclosed in U.S. Pat. No. 3,894,403 was developed for use in precision instruments to cool sample holders.